Combine the two medium place layers

Previously we had one place layer for capitals and one for non-capitals
when it came to towns and cities. This combines them, reducing the number
of queries, making the SQL simpler, and making the layers more useful for
other styles.

Incidentially fixes gravitystorm#522

placenames.mss

@@ -31,19 +31,21 @@
   }
 }
 
-#placenames-capital {
-  [zoom >= 5][zoom < 15] {
-    text-name: "[name]";
-    text-size: 10;
-    text-fill: @placenames;
-    text-face-name: @book-fonts;
-    text-halo-radius: 1.5;
-    text-min-distance: 10;
-    [zoom >= 6] {
-      text-size: 12;
-    }
-    [zoom >= 11] {
-      text-size: 15;
+#placenames-medium::capital {
+  [capital = 'yes'] {
+    [zoom >= 5][zoom < 15] {
+      text-name: "[name]";
+      text-size: 10;
+      text-fill: @placenames;
+      text-face-name: @book-fonts;
+      text-halo-radius: 1.5;
+      text-min-distance: 10;
+      [zoom >= 6] {
+        text-size: 12;
+      }
+      [zoom >= 11] {
+        text-size: 15;
+      }
     }
   }
 }

project.mml

@@ -1114,30 +1114,7 @@
       ],
       "Datasource": {
         "type": "postgis",
-        "table": "(select way,place,name,ref\n       from planet_osm_point\n       where place in ('city','town') and capital='yes'\n      ) as placenames_capital",
-        "extent": "-20037508,-19929239,20037508,19929239",
-        "key_field": "",
-        "geometry_field": "way",
-        "dbname": "gis"
-      },
-      "id": "placenames-capital",
-      "class": "",
-      "srs-name": "900913",
-      "srs": "+proj=merc +a=6378137 +b=6378137 +lat_ts=0.0 +lon_0=0.0 +x_0=0.0 +y_0=0.0 +k=1.0 +units=m +nadgrids=@null +wktext +no_defs +over",
-      "advanced": {},
-      "name": "placenames-capital"
-    },
-    {
-      "geometry": "point",
-      "extent": [
-        -179.99999692067183,
-        -84.96651228427099,
-        179.99999692067183,
-        84.96651228427098
-      ],
-      "Datasource": {
-        "type": "postgis",
-        "table": "(select way,place,name\n      from planet_osm_point\n      where place in ('city','town')\n        and (capital is null or capital != 'yes')\n      ) as placenames_medium",
+        "table": "(SELECT way,place,capital,name\n  FROM planet_osm_point\n  WHERE place IN ('city','town')) AS placenames_medium",
         "extent": "-20037508,-19929239,20037508,19929239",
         "key_field": "",
         "geometry_field": "way",